The invention relates to a method of analysing a specimen comprising a compound material by X-ray fluorescence analysis wherein a beam of polychromatic primary X-rays is generated in an X-ray tube by conversion of electric current into X-rays, and said beam is directed at the specimen in which specimen the primary X-rays are converted into chemical element specific fluorescent X-rays, and wherein the element specific fluorescent X-rays are selectively detected using means for detection and an intensity of said fluorescent X-rays is determined.
For the purpose of this application, a compound material is a material that comprises a mixture of different chemical elements, such as an alloy in the case that the material is a metal.
A method such as described above is known from U.S. Pat. No. 2,711,480. In the known method, a specimen of metal sheet comprising a backing and a thin layer covering the backing, is analysed using X-ray fluorescence analysis. During irradiation of the metal sheet with primary X-rays part of the primary X-rays are absorbed in the metal sheet and fluorescent X-rays are re-emitted by a chemical element that is comprised in the metal sheet. Some of these fluorescent X-rays are selectively detected. In order to determine the thickness of the thin layer, fluorescent X-rays re-emitted from a chemical element in the backing are selectively detected after they have passed through the thin layer. The fluorescent X-rays are partially absorbed in the thin layer. To determine the thickness of the thin layer, the measured intensity is compared to a reference intensity measured using a control specimen for the metal sheet whereon the thin layer is not present.
It is an object of the invention to provide a method for X-ray fluorescence analysis for determining at least the relative abundance of the chemical element in a specimen comprising a compound material. For the purpose of this patent application, the term relative abundance is used for the abundance of a chemical element in a compound material, expressed in percentage by weight.
It is an other object of the invention to provide a method for X-ray fluorescence analysis that can be combined in an easy way with the known method for determining the thickness of a thin layer covering a backing.
According to the invention, one or more of these objects is achieved by a method according to the first paragraph of this description, in which, after the electric current is applied to the X-ray tube and the intensity of element specific fluorescent X-rays is determined, a second intensity of the element specific fluorescent X-rays is determined while generating a second beam of primary X-rays using an electric current with a different value than the previous electric current, and at least the relative abundance of the chemical element present in the compound material is then determined using the values of both intensities.
The invention is based on the finding, that the ratio of the count rates in the case of two different tube currents during which application the X-ray fluorescence is detected, varies with the relative abundance (i.e. weight percentage) of the chemical element in the compound material. Therefore, the ratio of the determined intensities is a measure of the relative abundance of the chemical element in the compound material. Since this method of analysis comprises a relative measurement of intensities, the relative abundance of the chemical element can still be determined even though other factors may affect the absolute value of the intensities, such as the presence of an absorbing layer of material between the sheet and the means for detection.
It is believed that the ratio of the intensities of detected X-ray fluorescence in the case of two different tube currents, varies with the relative abundance of the chemical element in the compound material as a result of a change in the absorption characteristics of the initial polychromatic X-rays or the re-emitted fluorescent X-rays, or both. The relative abundance can easily be extracted from the measurement of the intensities by comparing to a calibration that is determined using control specimens of which the relative abundance of the chemical element has been established using an independent method such as a direct chemical analysis.
The second beam of primary X-rays may be generated using a second X-ray tube. However, it is preferred that the second beam of primary X-rays is generated with the same X-ray tube as the previous beam of primary X-rays. This is a cheap embodiment of the method. The one X-ray tube may be mounted stationary. This is relatively easy compared to instruments where angle of X-ray beam compared to the specimen can be varied, and furthermore there is no need to take into account any influence caused by different angles.
The method of analysis according to the invention can easily be combined with the known method for determining the thickness of a first layer on a second layer of material. In this case, the determined relative abundance of the chemical element is preferably used to calculate a reference X-ray fluorescence intensity to which at least one of the determined intensities is compared to determine the thickness of the first layer. Herewith it is achieved that the thickness of the first layer is determined by X-ray fluorescence analysis, without the need to separately measure the reference intensity of the control specimen without the first layer. The calibration contains the information that is required to calculate for each used X-ray tube electrical current the reference intensity of fluorescent X-rays once the relative abundance of the chemical element that is present in the specimen is determined.
In the case that the first layer comprises one or more sublayers, it is preferred that for each layer or sublayer in which the concentration of the chemical element is to be determined an additional intensity is determined of the selectively detected fluorescent X-rays while during this each time the electrical current is applied at a different value. From the thus determined intensities, it is possible to extract each relative abundance of the chemical element in each sublayer that is desired.
Preferably, a metal alloy is selected as the compound material, the metal alloy more preferably being an aluminium alloy, the aluminium alloy preferably containing a chemical alloy element of the group of Cu, Mn, Zn, Fe. Herewith the relative abundance of an alloying element that is often used within an aluminium alloy can be determined with a fast and cheap method that can be applied in a metal sheet production plant. Cu is frequently used as alloying element in aluminium products such as aluminium sheet. Cu is a fast diffusing element, that may redistribute in the product during various stages of production. Therefore, a method for non-destructive analysis of a specimen containing Cu is very important. The advantage of using the method according to the invention for analysis of Cu in aluminium sheet is that Cu is also a suitable X-ray fluorescence emitting element. For the purpose of this application, aluminium sheet is held to comprise aluminium alloy sheet.